Improve Chapter 31 legal issues lesson

[jules-tth]

Summary

• Rewrites the Chapter 31.9 legal issues lesson as a practical Linux operations guide.
• Covers software licensing, contracts, privacy/data protection, retention/deletion, legal holds, incident evidence, chain of custody, escalation paths, common mistakes, and a reversible evidence lab.

Duplicate prevention

• Verified PR Improve Chapter 31 compliance lesson #290 is the canonical Chapter 31 compliance slice.
• Final target-file overlap check returned no open PR touching docs/lessons/31_methodology_policy_and_politics/31.9_legal_issues.md immediately before PR creation.

Verification

• python3 -m mkdocs build --strict
• git diff --check
• make build